Following conventional practice in traction elevators, an elevator car approaching a floor for a "stop," the drive motor is slowed, the brake is dropped and, then the motor is stopped.
Motor torque should drop to zero simultaneously with brake engagement (dropping). Control inaccuracies and circuit characteristics may cause excessive residual torque when the brake engages. In some types of geared elevators, in which actual motor speed is fed back and summed with a dictated motor speed to produce an error signal used to provide a motor torque or speed signal, gear noise (chattering) is produced if the brake is engaged and motor torque is increased (by the sudden increase in the difference between actual motor speed, now zero, and the dictated motor speed, which has not changed) The gear noise or impact noise is of two forms. First, a chatter is caused by the difference between the actual motor speed and the dictated motor speed and thus the increase in motor torque; the chatter is caused because motor torque is increasing although the brake has already has applied. A second form of gear noise, a "thud", follows the chatter. Rather than being caused by increasing motor torque after the brake has been applied, a thud is caused by the torque current dropping instantaneously to zero such that the gear travels through a backlash.